Present-day mobile communication systems offer subscribers different data transmission features in addition to the conventional speech transmission. The data services of mobile communication systems can generally be divided into teleservices and bearer services. A bearer service is a telecommunication service, which provides signal transmission between user and network interfaces. For instance, modem services are bearer services whereas a telephone service and telefax service are teleservices. Data services are typically circuit-switched, which means that when using some services, a subscriber makes a call, during which data transmission takes place in the same manner as speech transmission during a speech call. However, mobile communication systems are not designed to take into account in all situations whether a circuit-switched connection is used for speech or data transmission but they handle different types of calls in the same manner, which causes problems in some situations.
One subscriber facility of the present-day mobile communication systems is call forwarding, which a subscriber can activate. Call forwarding may be unconditional call forwarding, a call being thus always forwarded to a given different number than that entered by the calling party. Call forwarding can also be arranged to take place conditionally, for instance when the subscriber is busy, the subscriber does not answer or the subscriber is not reached.
Another aspect of the present-day mobile communication systems is that a mobile exchange can switch audio-format information to a subscriber during different phases of a call. Audio information can be divided into announcements and audible tones. An announcement is a recorded or synthesized speech message. An announcement may be chargeable or charge-free to the calling subscriber. An announcement can also be switched to the calling subscriber during call set-up, in which case a so-called intermediate announcement is provided. For instance, in connection with call forwarding, the calling subscriber is informed of the forwarding of the call with a special intermediate announcement. Mobile communication network operators want to make this type of chargeable intermediate announcements of call forwardings, because otherwise, the functions of a network would be used free of charge in this type of situation in cases where neither the calling subscriber nor the subscriber of the call forwarding number are in the operator's network.
Intermediate announcements of this type are problematic in connection with data calls. Both a modem and a telefax modem wait for an answer signal under time-out control. The time-out of a telefax modem conventionally is 30-40 seconds, and the time-out of a modem is not defined in standards. The intermediate announcements made in connection with call forwarding may cause call set-up time to be prolonged to exceed the determined time limits, as a result of which the calling data modem or telefax modem assumes that the call attempt has failed and disconnects the call. As regards data calls, all long intermediate announcements are problematic, especially in connection with call forwarding on no reply. In addition, modems and telefax terminals can in no manner utilize the announcements, subscribers thus having to pay for unnecessary announcements. The same applies to all other data terminal equipments. The most unfortunate situation is where a subscriber is charged for a chargeable intermediate announcement, but the call set-up fails due to a delay caused by the intermediate announcement.
Thus, a need exists for preventing intermediate announcements from being made in connection with data calls. In the present-day mobile communication systems, this need has in no manner been taken into account, and the exchanges making intermediate announcements, for instance transit exchanges or terminal exchanges, do not at present even know the type of a mobile terminating call in connection with call forwarding, and they do not have any facilities for checking the type of call and for preventing intermediate announcements in connection with data calls.